1. Field of the Invention
This invention relates to conductive polymer compositions which exhibit PTC characteristics, and to devices comprising such compositions.
2. Summary of the Prior Art
It is known that polymers, including natural rubbers and other elastomers, can be made electrically conductive by dispersing therein suitable amounts of finely divided conductive fillers, e.g. carbon black. For a general survey of such materials (which are usually known as conductive polymers), reference may be made to "Conductive Rubbers and Plastics" by R. H. Norman, published in 1970 by Elsevier Publishing Co. It is also known that the electrical properties of conductive polymers frequently depend upon, inter alia, their temperature; and that a very small proportion of conductive polymers exhibit what is known as PTC positive temperature coefficient) behavior, i.e., a rapid increase in resistivity at a particular temperature or over a particular temperature range. The term "switching temperature" (usually abbreviated to T.sub.s) is used to denote the temperature at which the rapid increase takes place. When the increase takes place over a temperature range (as is often the case) then T.sub.s can conveniently be designated as the temperature at which extensions of the substantially straight portions of the plot of the log of the resistance against the temperature (above and below the range) cross. The resistance of PTC polymers continues to increase as the temperature rise above T.sub.s until it reaches a maximum, called the Peak Resistance, at a temperature which is called the Peak Temperature; the resistance thereafter decreases more or less rapidly.
Materials exhibiting PTC behavior are useful in a number of applications in which the size of the current passing through a circuit is controlled by the temperature of a PTC element forming part of that circuit. For practical purposes this means that the T.sub.s of the material should lie between about -100.degree. C. and about 250.degree. C. and that the volume resistivity of the material at temperatures below T.sub.s should be from about 25 to about 10.sup.5 ohm cm. The lower limit on resistivity results from the requirement that, at temperatures above T.sub.s, the PTC element should be an insulator; if the resistivity of the element below T.sub.s is less than 25 ohm. cm., then even after the increase in resistivity around and above T.sub.s, the resistivity will not be sufficiently high. The upper limit on resistivity results from the requirement that the PTC element should be a conductor at temperatures below T.sub.s. The practical effect of these limitations on resistivity is to exclude from consideration conductive polymers having either very high or very low loadings of conductive filler. Another practical requirement for PTC materials is that the increase in resistance above T.sub.s should be sufficiently high that the heater (or other device) is effectively converted from an electrical conductor to an electrical insulator by a relatively limited increase in temperature. A convenient expression of this requirement is that the material should have an R.sub.14 value of at least 2.5 or an R.sub.100 value of at least 10, and preferably an R.sub.30 value of at least 6, where R.sub.14 is the ratio of the resistivities at the end and beginning of the 14.degree. C. range showing the sharpest increase in resistivity; R.sub.100 is the ratio of the resistivities at the end and beginning of the 100.degree. C. range showing the sharpest increase in resistivity; and R.sub.30 is the ratio of the resistivities at the end and beginning of the 30.degree. C. range showing the sharpest increase in resistivity. A further practical requirement for most PTC materials is that they should continue to exhibit useful PTC behavior, with T.sub.s remaining substantially unchanged, when repeatedly subjected to thermal cycling which comprises heating the material from a temperature below T.sub.s to a temperature above T.sub.s but below the peak temperature, followed by cooling to a temperature below T.sub.s. It is also preferred that the ratio of the peak resistance to the resistance at T.sub.s should be at least 20:1, especially at least 100:1. It is further preferred that when T.sub.s is less than 150.degree. C. the ratio of the resistance to the resistance at T.sub.s should be at least 20:1 at 200.degree. C.; and when T.sub.s is greater than 150.degree. C., this ratio should be at least 20:1 at 250.degree. C.
Having regard to these practical limitations, it has been accepted in the art that in a conductive polymer composition exhibiting useful PTC behavior, the polymer must be a thermoplastic crystalline polymer, and that amorphous polymers are unsatisfactory for this purpose--see for example the article by J. Meyer in Polymer Engineering and Science, November 1973, 13, No. 6, pages 462-468. Thus PTC compositions comprising a thermoplastic crystalline polymer with carbon black dispersed therein have been widely used in self-regulating strip heaters. Such a composition shows a rapid increase in resistance over a range which begins at the softening point of the polymer and has a T.sub.s at or near the crystalline melting point of the polymer; the greater the crystallinity of the polymer, the smaller the temperature range over which the resistance increase takes place. Generally, the composition is cross-linked to improve its stability at temperatures above T.sub.s, and when this has been done in the past, the cross-linking has been effected by irradiation at room temperature, the polymer reaching a temperature of at most 45.degree. C.
PTC conductive polymer compositions in which the polymer is a thermoplastic crystalline polymer are very satisfactory for many purposes. However, it is a serious disadvantage that the T.sub.s and the physical properties of such compositions are governed by the polymer employed.
Although, as noted above, it has been accepted in the art that in conductive polymer compositions exhibiting useful PTC behavior, the polymer must be a thermoplastic crystalline polymer, there have been prior disclosures, usually in patent specifications, of PTC compositions in which the filter is dispersed in an amorphous thermoplastic polymer; and in which a filler is dispersed in a thermoset material; and in which a thermoplastic having the filler dispersed therein is distributed throughout another polymer which may be a thermoplastic, a synthetic or natural rubber, or a thermoset resin; and in which a conductive filler is dispersed in an elastomer. In many of these disclosures, it is clear that in all the compositions actually studied, the conductive filler was dispersed in a crystalline polymer, and that the T.sub.s of the composition was governed by the melting point of that polymer. In the few cases in which it appears that a conductive filler was dispersed in a non-crystalline polymer to prepare a PTC composition, the precise preparative details are often not clear enough to enable substantially similar compositions to be prepared independently; however, it does appear that in all such cases the compositions failed to exhibit useful PTC behavior as defined above and that the T.sub.s of the composition (if any) was associated with a thermodynamic transition point of the polymer or with the breaking of thermally labile bonds in the polymer.
For details of the prior disclosures referred to in the preceding paragraph, and of the conventional compositions comprising thermoplastic crystalline polymers, reference should be made to U.S. Pat. Nos. 2,978,665; 3,243,753; 3,412,358; 3,591,526; 3,793,716; 3,823,217; and 3,914,363; British Pat. No. 1,409,695; Brit. J. Appl. Phys, Series 2, 2, 567-576 (1969, Carley Read and Stow); and Kautschuk und Gummi II WT 138-148 (1958, de Meij); as well as the Meyer article referred to above, the disclosures of which are hereby incorporated by reference. For details of recent developments in this field, reference may be made to U.S. patent applications Ser. Nos. 601,638 (now U.S. Pat. No. 4,177,376, 601,427, 601,639 (now abandoned), 601,424 (now abandoned), 601,549 (now abandoned), and 601,344 (now abandoned) (all filed Aug. 4, 1975), 638,440 and 638,687 (now abandoned) (both filed Dec. 8, 1975) and the application, having Serial No. 706,602, (now abandoned) filed July 19, 1976 by Kamath and Leder and entitled "Improved PTC Strip Heaters", the disclosures of which are hereby incorporated by reference.
There are many uses for conductive polymers for which PTC behavior is of no significance, or at least of no recognised significance, and in many of them, e.g., the prevention of static charges in floor coverings, etc., the conductive polymer is not in contact with an electrode. Many different conductive polymers have been proposed for such uses, for example natural rubbers, which may be sulfur-cured after incorporation of the conductive filler. In some cases particular polymers are in practice invariably used with particular conductive fillers and cured in particular ways. For example, polysiloxanes are in practice invariably rendered conductive by incorporation of acetylene black and peroxide-cured at relatively high temperatures, e.g., 170.degree.-180.degree. C.
As will further be made clear hereinafter, a very careful selection and combination of features is needed in order to obtain a useful PTC composition in which the filler is dispersed in a cross-linked elastomer, and so far as we are aware, useful PTC behavior is not an inherent property of any of the many conductive cross-linked elastomers which are known for uses in which PTC behavior is of no significance, or at least of no recognised significance.